Sulphur compounds are naturally present in wastewater, more in particular industrial wastewater. There are many problems linked to the presence of sulphur compounds in wastewater, such as:                the emission of nauseating odours (rotten egg type). These unpleasant odours are detected well before they represent a danger to human beings since the olfactory sensors have a detection threshold at 0.15 ppm.        the health risks related to hydrogen sulphide. In fact, this molecule is toxic starting from 10 ppm, with worsening of the effects according to the duration of exposure.        corrosion of concrete and metals due to oxidation of sulphides to sulphuric acid by certain bacteria of the Thiobacillus type.        the development of filamentous bacteria. Since the water is septic, filamentous bacteria are more competitive than the conventional bacteria of the flock with respect to oxygen. Some are capable of accumulating sulphur in the form of granules in their cells.        the need to cover the works in order to prevent dispersion of hydrogen sulphide.        
The treatment of industrial wastewater, with a focus on removal of sulphide compounds, can be done chemically, biochemically, as well as biologically.
Established chemical processes are precipitation with iron salt and oxidation with oxidizers like hydrogen peroxide. The disadvantage of precipitation with iron salt is that the solids need to be separated, while the disadvantage of using oxidizers is that these are expensive and partly hazardous to handle.
Examples of biochemical processes for the treatment of wastewater include biochemical treatment in anoxic basins and biochemical solid bed processes. The disadvantage of using anoxic basins is that they take in a large volume as the sludge comes in as a suspension. For that reason, such systems using anoxic basins are expensive to build. The disadvantage of biochemical solid bed processes is that over time, clogging occurs.
Today, there is however the tendency to choose for biological wastewater treatment using microorganisms, mostly including bacteria and protozoa. Typically, the wastewater is mixed with activated sludge on which special types of microorganisms are cultivated. The term “activated” means that there is biological activity. Mostly, the sludge needs to be pushed with an inoculum injection in the beginning to produce the correct microbiology. The microorganisms are able to decompose the pollutants present in the wastewater and convert them into biomass.
At present, aerobic as well as anaerobic sludge systems are known. Anaerobic processes convert soluble organic carbon into carbon dioxide and methane (=one of the main components of biogas), in contrast to aerobic systems, which only produce carbon dioxide.
In aerobic sludge systems, air or oxygen is introduced into a mixture of screened, and primary treated wastewater combined with microorganisms to develop a biological flocculation which is able to degrade certain undesired compounds in the wastewater. To maintain aerobic conditions and to the keep the active biomass suspended, a constant and well-timed supply of oxygen is required. Also, the oxidation of sulphide to sulphur or sulphate by injection of air demands sufficient oxygen uptake of the wastewater sludge. Due to the low solubility of oxygen in water, the oxidation capacity and thus the room specific performance (=removal of sulphide per m3 reactor) is low. In addition, the nitrogen in air leads to stripping of sulphide and thus increased emissions of non-oxidized sulphide. Consequently, air based oxidation processes for removal of sulphide have strong limitations.
Over the past several years, anaerobic methods have been increasingly used for treatment of amongst others industrial wastewater to remove suspended and soluble organic matter from these aqueous wastewater streams. Anaerobic activated sludge systems use anaerobic microorganisms that treat wastewater in the absence of oxygen. The term “anaerobic” thus refers to the bacterial metabolism that occurs in the absence of oxygen. If anaerobic processes are correctly controlled, they can lead to a high level of purification of wastewater.
Significant disadvantages of aerobic wastewater treatment processes over anaerobic wastewater treatment processes are that aerobic processes require large amounts of oxygen and larger volumes for oxygen transfer, through which the aerobic wastewater treatment systems are less cost effective.
Other advantages of anaerobic wastewater treatment processes over aerobic wastewater treatment processes are the following:                during the anaerobic treatment process, an amount of valuable biogas energy will be produced which can be collected for other usage;        much less bio-solids waste generated compared with aerobic process because much of the energy in the wastewater is converted to a gaseous form and resulting in very little energy left for new cell growth;        a low energy requirement for the anaerobic treatment process;        less nutrients are required;        anaerobic wastewater treatment systems can be shut down for extended periods without serious deterioration; and        the anaerobic wastewater treatment process can handle organic shock loads effectively.        
Amongst sludge treatment systems, suspended sludge as well as granular sludge treatment systems are known.
Granular sludge treatment systems comprise a bioreactor containing sludge granules. These sludge granules are aggregates of microorganisms that are formed during wastewater treatment in an environment with a constant flow hydraulic regime. In the absence of any support matrix, the flow conditions create a selective environment in which only those microorganisms, capable of attaching to each other, survive and proliferate. Finally, the aggregates form into dense compact biofilms referred to as granules. Due to their large particle size, generally ranging from 0.5 to 2 mm, the granules resist washout from the reactor, permitting high hydraulic loads.
Likewise granular sludge treatment systems, suspended sludge treatment systems also use microorganisms. However, in the suspended sludge treatment systems, these microorganisms form rather small flakes in the wastewater or sludge, through which a suspension is formed in the wastewater. The microorganisms and the wastewater thus form a slurry. Granular sludge treatment systems provide an increased biological activity compared to suspended sludge treatment systems due to giving more protection for the microorganisms in the granules. The granular sludge treatment system furthermore provides a far easier separation of the sludge and the liquid phase and after the wastewater has been treated.
Up-flow Anaerobic Sludge Blanket (UASB) digestion reactors are known for stable and efficient anaerobic degradation and biogas production with a high concentration of methane which is formed as a by-product. UASB digestion reactors use an anaerobic process whilst forming a blanket of granular sludge which suspends in the reactor. Wastewater flows upwards through the blanket and is processed (degraded) by anaerobic microorganisms. The upward flow combined with the settling action of gravity suspends the blanket with the aid of flocculants. The blanket begins to reach maturity at around 3 months. Small sludge granules begin to form whose surface area is covered in aggregations of microorganisms. In the absence of any support matrix, the flow conditions creates a selective environment in which only those microorganisms, capable of attaching to each other, survive and proliferate. Eventually, the aggregates form into dense compact biofilms referred to as “granules”. Generally, during the treatment of UASB reactor, the substrate passes through an expanded sludge bed which containing a high concentration of biomass first. After that, the remaining part of substrate passes through a less dense biomass which named the sludge blanket. The UASB digestion-technology needs constant monitoring when put into use to ensure that the sludge blanket is maintained, and not washed out (thereby losing the effect). An example of such an UASB reactor is described in amongst others EP 2669255 and WO 2007/048537.
A known variant of the UASB concept for anaerobic wastewater treatment is an expanded granular sludge bed (EGSB) reactor. Both the UASB reactor and the EGSB reactor make use of granules, but differ in term of geometry, process parameters and applications. The distinguishing feature between the EGSB reactor and the UASB reactor is that a faster rate of upward-flow velocity is designed for the wastewater passing through the sludge bed. This increased flux permits partial expansion (fluidisation) of the granular sludge bed, improving wastewater-sludge contact as well as enhancing segregation of small inactive suspended particles from the sludge bed.
A down-flow alternative for the UASB and the EGSB reactor is a static granular bed reactor (SGBR). The SGBR includes a fixed bed of anaerobic granules in a down-flow configuration without flow recirculation. An example of such an SGBR is described in U.S. Pat. No. 6,709,591. The SGBR uses a down-flow bioreactor that is filled with active anaerobic granular biomass. Influent wastewater is distributed evenly across the bioreactor and passes downward through the granules. The gas that is produced by the granules provides channelization of the bed to prevent clogging. Clogging may also be prevented by recirculation of the gas or effluent to dislodge any trapped granules.
Because of the advantages as mentioned above, the invention relates to an anaerobic granular sludge treatment system for treatment of wastewater, focused to be used for the reduction of sulphur compounds in sulphur compound contaminated wastewater streams.
In the use of anaerobic granular sludge treatment systems, there exists the need to provide an anaerobic granular sludge treatment system that provides in a better recovery of the elemental sulphur or sulphate obtained by the oxidation of the sulphur compounds in the wastewater stream without disturbing the biological part of it.